A) Field of the Invention
The present invention relates to a semiconductor device and its manufacture method, and more particularly to a semiconductor device having shallow trench isolation (STI) and its manufacture method.
B) Description of the Related Art
The local oxidation of silicon, LOCOS, has been used as one of the element isolation methods for semiconductor devices. The local oxidation of silicon is the techniques wherein a silicon oxide layer is formed on a silicon substrate as a buffer layer, thereafter a silicon nitride film is formed as an oxidation preventing mask layer, the silicon nitride layer is patterned and thereafter the surface of the silicon substrate is thermally oxidized.
While the silicon substrate is thermally oxidized, oxidation seeds such as oxygen and moisture invade also into a buffer oxide silicon layer under the end portion of the silicon nitride layer and oxidize the silicon substrate surface under the end portion of the silicon nitride layer, so that a silicon oxide region of a bird's beak shape called a bird's beak is formed. The region where the bird's beak is formed cannot be used substantially as the element forming region (active region) so that the element forming region is narrowed.
If a silicon nitride film having openings of various sizes is formed and the substrate surface is thermally oxidized, the thickness of a silicon oxide layer formed on the silicon substrate surface with the small opening size is thinner than the silicon oxide layer formed on the silicon substrate surface with the large opening size. This phenomenon is called thinning.
As miniaturization of semiconductor devices advances, the ratio of an area not usable as the element forming region to the whole area of a semiconductor substrate is increased by bird's beaks and thinning. Namely, the ratio of narrowing the element forming region increases, hindering the high integration of semiconductor devices.
As techniques of forming an element isolation region, trench isolation (TI) techniques are known wherein a trench is formed in the surface layer of a semiconductor substrate and insulating material or polysilicon is buried in the trench. This method has been applied to bipolar transistor LSIs which require a deep element isolation region.
Application of the trench isolation to MOS transistor LSIs has progressed because both the bird's beak and thinning will not occur. Element isolation can be realized by a relatively shallow trench of about 0.1 to 1.0 μm in depth because MOS transistor LSIs do not require a deep element isolation like bipolar transistor LSIs. This structure is called shallow trench isolation (STI).
U.S. Pat. No. 5,447,884 discloses that a pad oxide film and a pad nitride film are formed on a silicon substrate, a trench is formed by anisotropic etching by using a resist pattern, a trench inner wall is thermally oxidized, thereafter a silicon nitride layer liner having a thickness of 5 nm or thinner (a thickness preventing hot phosphoric acid from invading) is formed, a burying silicon layer is deposited on the silicon nitride layer and the pad nitride film is removed by hot phosphoric acid.
Japanese Patent Laid-open Publication No. HEI-11-297812 discloses that a trench for element isolation is formed in a silicon substrate, a silicon oxide layer and a silicon nitride liner are formed on the trench inner wall, and the upper portion of the nitride silicon liner is removed to lower the upper end of the silicon nitride layer.
“2003 Symposium on VLSI Technology Digest of Technical Papers” by K. Goto et al reports that in the structure that the upper end of a silicon nitride film liner formed on the inner wall of an element isolation trench is lowered and a contact etch stopper layer of a silicon nitride film is formed on the silicon substrate, covering the gate electrode, the drive current of a MOS transistor can be improved by controlling the stresses in both the silicon nitride films.
Although the element isolation by STI is suitable for element miniaturization, there arises the problems specific to the STI itself. Novel techniques has long been desired which can mitigate the problems specific to the STI itself.